1. Field of the Invention
The invention relates to compression gauges in general and to compression gauges for use in combination with hydraulic wrenches in particular.
2. Prior Art
Hydraulic wrenches are well known in the art. Examples can be seen in U.S. Pat. Nos. 6,553,873 and 6,408,720, which are hereby incorporated by reference in their entirety. Such wrenches are frequently used to apply significant torque to large bolts such as the ones used to connect riser joints used in offshore petroleum structures. In designing these joints, engineers determine a compression that the bolts must exert in order to secure the joint. It is important that a degree of compression close to the desired level is applied. Generally, most design parameters call for tolerances within about ten percent. If insufficient compression is obtained, the riser may be able to flex excessively at the joint, which can lead to bolt fatigue which in turn can lead to joint failure. The compression exerted by a bolt is a direct result of the tension placed in the bolt. Generally, the design specifications are such that the bolts should be subjected to no more than about sixty percent of the tension necessary for the bolt to fail. If excess tension is placed on the bolt, the bolt may be excessively distorted which can result in the failure of the bolt under the operational load applied to the riser. Failure of these bolts can lead to joint failure and loss of the riser. Thus, it is important that the correct amount of compression be obtained.
Design engineers often convert the desired compression, typically measured in pounds of force, into torque to be applied by the wrench, typically measured in foot-pounds. This creates problems because the torque exerted by the wrenches is dependent upon conditions that can vary such as the rig hydraulics and the dimensions of the wrench being used to tighten the bolt. Moreover, tool operators usually cannot directly assess how much torque they are applying. Rather, they can tell the amount of hydraulic pressure (in pounds per square inch or psi) they are applying to the wrench. The hydraulic pressure must be converted to torque which is itself a conversion from compression. Thus, two steps are required to convert the measurable quantity, pressure, into the desired quantity, compression. While this is usually done beforehand, it still creates an opportunity for error that can have significant adverse consequences. Therefore, a device meeting the following objectives is desired.